Various devices for measuring the flow of blood through a body tissue are known from the prior art. To measure the flow of blood through the cerebral tissue of the brain, catheter-type measurement devices are used which carry measurement sensors at their catheter tip. Such catheter probes are inserted into the cerebral tissue though an opening made in the cranium, in order to carry out a measurement of the flow of blood through the tissue. A number of measurement methods are known for this purpose, for example thermal diffusion, ultrasound methods, and near-infrared spectroscopy in association with an indicator.
For example, EP 1 464 276 A1 discloses a measurement device for determining the flow of blood, in which device two octodes are placed on the surface of the head at a distance from each other. One of the octodes is connected to a radiation source which emits radiation with a near-infrared wavelength. Some of the radiation reflected on the cerebral tissue strikes the second octode, such that the intensity can be determined by an evaluation unit. Indocyanine green is used as the indicator, and a beam of light at a wavelength of between 780 and 910 nm is used. In this method of performing a near-infrared spectroscopy measurement, account must be taken of a large number of external influences that have a disadvantageous impact on the measurement of the flow of blood. The beam of light cannot be conveyed directly to the tissue that is to be measured, and instead it first has to pass through the skin, the skull cap, the dura mater, etc., in order to reach the tissue that is to be examined. As a result, the measurement signal is weakened and distorted by absorption and scattering, for example. It is therefore only possible to measure areas of tissue near to the surface of the head. Areas in the interior of the brain, for example near the floor of the ventricle, can be examined only with inadequate precision by this method.
EP 1 504 715 discloses a catheter with a light-emitting optical conductor and a light-receiving optical conductor, the ends of which optical conductors are arranged at a predetermined distance from each other.
Another device for measuring the flow of blood within the cranium is known from U.S. Pat. No. 5,579,774, for example. A catheter probe, inserted into the interior of the brain, comprises a measurement sensor for carrying out laser Doppler flowmetry. The beam of light of a helium-neon laser at 632.8 nm is guided in the axial direction via a light conductor to the measurement area, which lies distally in a continuation of the probe. Some of the incident light is absorbed and reflected by the surrounding tissue and some by the circulating blood. The reflected light is guided through at least one optical fibre to a processing unit. The light reflected from the moving red blood cells undergoes a Doppler shift, from which it is possible to determine the flow rate. The head of the catheter probe has a rounded tip whose diameter widens conically in the distal direction. The blunt tip is pushed through the opening in the cranium and through the subjacent tissue as far as the measurement area lying within the brain. The whole surface of the blunt tip presses against the cerebral tissue and exerts a pressure that can leave behind permanent damage in the brain.